The invention relates generally to water traps or liquid sepators, and more particularly, is directed to a liquid separator that solves unique problems associated with removing water, liquids and other debris from the sample gas in a gas analyzer.
Both dispersive and nondispersive radiant energy gas analyzers are used in the prior art to measure certain predetermined constituents of a sample gas containing contaminants such as water, which interfere with the operation of the analyzer. For example, nondispersive type infrared gas analyzers are currently used for breathing gas analysis, both in the medical field and the law enforcement field. In the medical field, such analyzers are used to determine the concentration of a wide variety of gases in a patient's breath. For example, the art of capnography involves the precise measurement of carbon dioxide concentrations in the breathing gas of a patient. This data, if accurate, is quite useful in determining a patient's ventilatory status as well as other physiological conditions. In the law enforcement field, nondispersive analyzers have been used for determining ethanol concentration in the breathing gas expelled from a driver's lungs. In both of these applications, water, liquids and other debris are often expelled from the subject along with breathing gases.
In the past, attempts to remove liquids from the sample gas flow in such analyzers have generally involved the use of a simple water trap comprising a cannister or housing substantially filled with a water absorbing material. The housing is provided with an inlet and an outlet for passing sample gas through the housing and the absorber material contained therein. The problem with this type of water trap stems from the detrimental effect that it has on the response time of the analyzer and its limited capacity. In the art of breath-by-breath analysis, the available volume of sample gas can be quite small and this type of water trap impairs the response time of the analyzer because of its relatively large volume and the mixing of sample gas in the trap. When the size of the water trap housing is reduced to a point at which the volume of the housing is low enough to have a negligible effect on the response time of the analyzer, the amount of absorbent retained within the housing no longer provides a satisfactory water absorbing capacity.
This problem is found to be particularly acute in the field of capnography, where it is important to obtain a continuous and very accurate measure of the carbon dioxide content in the end tidal portion of a patient's breathing gases. Such analyzers are often referred to as capnographs. While the volume of a normal adult patient's lungs is substantial, only a portion of the breath is analyzed, namely, the last portion of the breath expelled or end tidal portion. This problem is, of course, exacerbated in children and neonatals.
High levels of secretions are encountered in patients that are anesthetized or patients that are in critical care situations. These patients are normally supplied highly humidified gases. The condition of the patient is often such that mucous, sputum and/or blood can be present in the sample tube as well as condensed water. The sample tubes are small because the sample is small and thus blockages from such high viscosity liquids are common. Heating of the sample tube to prevent condensate from interfering with the operation of the analyzer further promotes the build-up of these highly viscous liquids. Such blockages interfere with gas flow and flow profile. This is intolerable in a capnograph which must be reliable, accurate and highly responsive.
The misnomer "water trap" is often applied to liquid collection devices that are used to strip contaminants from the sample flow in a capnograph. If water was the only fluid to contend with then other solutions are possible. In the past, large area collection vials (discussed above) or tortuous path water traps have been used to deposit or accelerate liquid particles out of the sample gas flow, respectively. The large area fluid traps are effective in separating liquids from sample gas but so greatly reduce the response time of the capnograph that physiological data cannot be truly recorded (i.e. transient changes in respiratory gas are completely damped). The tortuous path traps function relatively well on entrained pure water, but high viscosity fluids are not easily accelerated out of the gas path.
While most analyzers used in capnography to determine carbon dioxide concentration are relatively simple and cost effective, the efficacy of these instruments has sparked a continuous debate among physicians because of slow response and frequent failures. Monitoring of patients in the operating room or critical care situations is desirable, but physicians have, through experience, developed reservations concerning their use because of the failure reputation of these instruments.